The US-Mexico border region is an important and understudied area for research on disparities in health care access, quality, and cost. Many US citizens and legal residents from the border region cross into Mexico for medical treatment and/or pharmaceuticals. Border crossing is common due to inadequate health care services on the US side of the border, cultural preferences, looser prescription requirements, and more affordable provider options (1-5). Since 2006, violent crime has risen rapidly in the northern Mexican states;this violent crime is likely to increase the cost of crossing the border for health care and decrease the likelihood of seeking care in Mexico. No prior studies have addressed the impact of crime in Mexico on border crossing for health care;it is not known whether US residents will forego care or will substitute Mexican care with US providers. I will measure indirectly which of these is occurring by estimating changes in identifying a regular provider (Aim 1). Foregoing ambulatory care altogether may lead to increases in inpatient admissions for ambulatory care sensitive conditions (Aim 2). A change in utilization of emergency departments by the uninsured may be due to either substitution of US care (e.g., using the ED for non-emergent conditions) or foregoing care (e.g., presenting to the ED with a serious condition because of delay to care), which will be studied in Aim 2. If the analysis for Aim 1 shows that violence affects the likelihood of having a regular provider, I will use instrumental variables to estimate the effect of reporting a regular provider on rates of ACS conditions at the county level (Aim 3). For Aims 1 and 3, I will use individual data from the Behavioral Risk Factor Surveillance System. For Aims 2 and 3, I will use discharge data maintained by AHRQ's Healthcare Cost and Utilization Project;I will rely on State Inpatient Databases as well as State Emergency Department Databases to measure admissions for ambulatory care sensitive conditions as well as non-urgent and preventable use of the emergency department. Estimating changes in US health care as a result of rising crime in Mexico is important to determine appropriate policy responses to this ongoing problem. Quantifying the effect of reporting a regular provider on rates of ambulatory care sensitive admissions is a question that is of interest not only in the border region, but for the rest of the US health care system as well. PUBLIC HEALTH RELEVANCE: Rapid changes in health care access caused by increased crime in northern Mexican states will likely have an impact on health care utilization and quality in the US border region. Estimates of this effect will help shape policy responses to this ongoing violence to maintain or improve health care access and health outcomes in the border region.